well first of all divide 56 by two to get 28. now you have o find the two numers that square to 400 and add to 28. now im pretty sure you can do this with an equation but i forgot it and thus did guess and check and found the dimentions to be 12 and 16. PROOF: 12+16=28 and 12 squared is 144, 16 squared is 256, 144+256=400